Who cares about realism or fantasy? It is merely the choice of the theme. What
is more important, is being internally consistent, and internally logical. You
choose a set of facts and from there on try to do what seems "logical".
An example of breaking "internally consistent rules of logic" from the movie
"signs".
A vastly superior race of space-beings (oh my god, not realistic!!) come to
earth with their superior technology, and.. tadam, they CAN'T get past a
simple wooden DOOR with all their superior tech!

Fuck realism - we'we chosen fantasy. But there are still guidelines of
reasonability.